Orthodontic device

ABSTRACT

Disclosed is an orthodontic appliance including a barrel housing a main spring secured at opposite ends to the barrel and to a pulley rotatably carried by the barrel. A pull cable is wrapped about the pulley and exits the housing tangentially through a guide tube for attachment to a band about the tooth undergoing correction. A wire connected to the housing substantially at a location lying along the tangent on which the pull cable leaves the housing but on the opposite side of the radius of such tangent is coupled to an anchor tooth whereby the barrel is suspended intraorally between an anchor tooth and the tooth undergoing correction. The pull cable carries a stop whereby the mainspring is initially set to apply a predetermined tension on the pull cable. When secured to the anchor tooth and the tooth undergoing correction, and with the pull cable extending from the housing, the spring force is applied to the tooth undergoing correction moving it in the desired manner. In another embodiment hereof, the pulley is provided with a cam surface about its periphery which increases in radius as the pull cable is extended from the pulley whereby a substantially constant tension is provided between the anchor tooth and the tooth undergoing correction throughout the range of movement of the pull cable and said tooth.

United States Patent James [111 3,921,295 1451 Nov. 25, 1975 ORTHODONTIC DEVICE {75] Inventor: John Oliver James, Lancaster, Pa.

[73] Assignee: Hamilton Technology, Inc., Del.

[22] Filed: Aug. 26, 1974 [21] Appl. No.: 500,504

52] us. 01 .L 32/14 E [51] Int. Cl. A61C 7/00 [58] Field of Search 242/1072; 32/14 A, 14 E [56] References Cited UNITED STATES PATENTS 3,416,229 12/1968 Kesling 32/14 A 3,654,702 4/1972 Kelly 32/14 A Primary ExaminerRobert Peshock Attorney, Agent, or Firm-LeBlanc & Shur [57] ABSTRACT Disclosed is an orthodontic appliance including a barrel housing a main spring secured at opposite ends to the barrel and to a pulley rotatably carried bythe barrel, A pull cable is wrapped about the pulley and exits the housing tangentially through a guide tube for attachment to a band about the tooth undergoing correction. A wire connected to the housing substantially at a location lying along the tangent on which the pull cable leaves the housing but on the opposite side of the radius of such tangent is coupled to an anchor tooth whereby the barrel is suspended intraorally between an anchor tooth and the tooth undergoing correction. The pull cable carries a stop whereby the mainspring is initially set to apply a predetermined tension on the pull cable. When secured to the anchor tooth and the tooth undergoing correction, and with the pull cable extending from the housing, the spring force is applied to the tooth undergoing correction moving it in the desired manner. In another embodiment hereof, the pulley is provided with a cam surface about its periphery which increases in radius as the pull cable is extended from the pulley whereby a substantially constant tension is provided between the anchor tooth and the tooth undergoing correction throughout the range of movement of the pull cable and said tooth.

11 Claims, 7 Drawing Figures US. Patent Nov. 25, 1975 Sheet10f3 3,921,295

U.S. Patent Nov.25, 1975 Sheet2of3 3,921,295

US. Patent Nov.25, 1975 Sheet3 of3 3,921,295

w A fi 720 ORTHODONTIC DEVICE The present invention relates to an orthodontic appliance and particularly relates to an intraoral orthodontic appliance having a mainspring for applying a tension force between an anchor tooth and a tooth undergoing correction.

During orthodontic treatment, it is desirable to exert a force between an anchor tooth and a tooth undergoing correction to cause movement or translation of the latter tooth. Ideally, a tooth will move most efficiently if a constant force is continuously exerted during its entire movement. In certain prior appliances, this force has been provided by rubber bands or by extension springs. The durability of rubbermakes the former solution undesirable while the conventionally utilized springs as well as the rubber bands rapidly decay in force in response to limited movement of the tooth undergoing correction. Since the force applied by conventional springs is directly proportional to their linear extension, frequent manipulation of the appliance by the orthodontist is necessary to correct this continually changing force to the desired force. Thus such prior extension springs are not ideally suitable for use in orthodontic appliances. An attempt to provide a constant or noncumulative force spring in an orthodontic appliance is disclosed in US. Pat. No. 3,654,702. In that patent, however, the force applied to the tooth undergoing correction appears uncontrolled in magnitude and does not provide for differentiation between various magnitudes of forces required in orthodontic procedures.

The present invention provides a novel and improved orthodontic appliance which minimizes or eliminates the foregoing and other problems associated with prior orthodontic appliances and provides a novel and improved orthodontic appliance having various advantages in construction, mode of application and use in comparison with such prior devices. Particularly, the present invention provides an orthodontic appliance which will provide a near-constant tension force between the anchoring tooth and the tooth undergoing correction throughout the small discrete range of movement necessary to correct the tooth and which appliance is provided either singly or as one ofa set of appliances having various spring forces. In this manner, the correct magnitude of force can be applied while obtaining a substantially near constant spring force throughout the limited range of movement of the tooth undergoing correction. To accomplish this, the present invention in one form hereof provides a barrel housing a mainspring and a pulley, the opposite ends of the mainspring being secured to the housing and pulley to bias the pulley for rotation in one direction. A pull cable is wrapped about the pulley and is received through a guide in the barrel. The pull cable carries a stop whereby a predetermined prewind and hence tension force on the pull cable is set into the appliance prior to use. A securement is provided on the barrel for securing the barrel to a wire or the like in turn secured to the anchor tooth. This securement is located substantially along the tangent of the pull cable as it extends from the pulley but on the opposite side of the radius of such tangent. Thus, when the appliance is applied to the teeth, the barrel, mainspring and pulley, are suspended between the anchor tooth and the tooth undergoing correction. In this configuration, the mainspring applies a continuous, substantially near-constant tension force to the tooth undergoing correction along a line from the anchor tooth, coincident with the wire connecting the anchor tooth and the barrel, and along the barrel per se and pull cable.

With the foregoing arrangement, only a very limited range of linear travel is set into the appliance whereby a near constant tension is provided along the pull cable. Particularly, the pull cable is wrapped about the pulley for less than its full circumference whereby the linear travel is limited to a discrete increment. The mainspring provides a near constant force throughout this discrete range of movement although a small force gradient does obtain. With the foregoing arrangement, sets of such appliances can be provided with variously sized mainsprings and degrees of prewind to provide a range of tensions on the pull cable. each appliance having a substantially near-constant tension throughout the range of its particular movement;

In another form hereof, a substantially constant tension force is provided by the pull cable. In this form, the appliance is substantially as described above except that a cam surface is provided about the periphery of the pulley. Particularly, the cam surface is formed to increase in radius about the pulley as the pull cable is drawn from the pulley. Accordingly. since the tension on the pull cable is a function both of the strength of the mainspring and the working radius of the pulley groove, the normal force gradient provided by the mainspring as the pull cable is extended from the housing is substantially offset by the decrease in force afforded by extending the cable from a periphery which increases in radius as the pulley rotates against the main spring bias. This results in a reduction of the normal force gradient over the discrete range of movement of the spring but does not eliminate the force gradient entirely.

Accordingly, it is a primary object of the present invention to provide a novel and improved orthodontic appliance.

It is another object of the present invention to provide a novel and improved intraoral orthodontic appliance for providing a substantially near constant force between an anchor tooth and a tooth undergoing correction and throughout a limited range of movement of the latter tooth.

It is still another object of the present invention to provide a novel and improved intraoral orthodontic appliance provided as one of several sets thereof with each appliance in the set having a predetermined linear tension force different than the predetermined linear tension forces of the other appliances of the set.

It is a further object of the present invention to provide a novel and improved intraoral orthodontic appliance for providing a substantially constant force between the anchor tooth and the tooth undergoing correction by reducing the normal force gradient over the discrete range of movement of a mainspring.

It is a still further object of the present invention to provide a novel and improved intraoral orthodontic appliance wherein the appliance is uniquely secured between the anchor tooth and the tooth undergoing correction.

It is a related object of the present invention to provide a novel and improved method of applying an intraoral orthodontic appliance.

These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims 3 and drawings wherein:

FIG. 1 is a side elevational view of an orthodontic appliance constructed in accordance with the present invention and illustrated in operative position for correcting a tooth;

FIG. 2 is an enlarged cross-sectional view of the barrel thereoftaken generally about on line 2-2 in FIG. 1;

FIG. 3 is an enlarged cross-sectional view taken generally about on line 3-3 in FIG. 3;

FIG. 4 is an enlarged persepctive view of the ele ments forming the orthodontic appliance hereof, such elements being illustrated in exploded juxtaposition.

FIG. 5 is a plan view of another form of orthodontic appliance constructed in accordance with the present invention; and

FIGS. 6 and 7 schematically illustrate the eccentricity of the pulley groove relative to the pulley whereby the force gradient remains substantially constant during retraction of the pull wire.

Referring now to the drawings. and particularly to FIG. 1, there is illustrated an orthodontic appliance generally designated 10 comprised of an anchor wire 12 coupled to an anchor tooth 14, a pull wire 16 coupled to a tooth 18 undergoing correction, and an appliance housing 24 tensioned therebetween. Conventional orthodontic bands 20 and 22 are clamped about the respective anchor tooth 14 and the tooth 18 subject to correction and are connected conventionally to the respective anchor wire 12 and pull wire 16. The opposite end of the pull wire 16 extends within the appliance housing 24 and is wrapped about a pulley 26 biased for rotary movement in one direction by a mainspring 28, the details of which will now be described.

Appliance housing 24 comprises a generally cylindrical casing or barrel closed on one side by a wall 30 and open through its opposite side. Projecting axially inwardly from side wall 30 about the central axis of barrel 7 24 is a sleeve or hub 32, the central opening 34 of which secures the elements of the appliance one to the other. A pair of cars or lugs 36 and 38 project outwardly from the generally cylindrical exterior surface of barrel 24 substantially along the open side of barrel 24 as best illustrated in FIG. 4. Ear 36 is provided with an opening 40 to facilitate securement between anchor wire 12 and the appliance housing 24. Ear 38 is provided with a passage 42 in communication with the interior of barrel 24, passage 42 lying generally along a tangent to the cylindrical inside surface thereof and through which passage pull wire 16 is received for engagement about pulley 26. For reasons described hereinafter, a recess 44 is formed along a portion of the generally circular interior surface which defines with pin 32 an annular chamber 46.

As best illustrated in FIG. 2, pulley 16 is provided with a radially recessed groove 48 inset from its outer periphery for receiving a portion of pull wire 16. The end of pull wire 16 is suitably secured within groove 46, for example by staking the same in groove 48 as illustrated at 50. Pulley 26 has an axially projecting stem 52 which is preferably axially slotted at a plurality of circumferentially spaced positions, four slots being illustrated in FIG. 3. The central bore 56 defined by stem 52 extends throughout its length and is counterbored at 58 along the outer surface of pulley 26. Preferably, pulley 26 is formed of stainless steel.

An annular plate 60 is provided. Particularly, plate 60 is adapted to seat on an annular shoulder 62 formed about the inside surface of barrel 24. Mainspring 28 4 comprises a substantially conventional stainless steel mainspring with the inner end of the mainspring bent inwardly or inturned as at 64 for reception within one of the slots 54. The outer end 66 of mainspring 28 is reversely bent outwardly upon itself for reception in recess 44 formed on the inside surface of barrel 24.

It will be appreciated from the foregoing description of the elements of orthodontic appliance 10 that main spring 28 is received within barrel 24 with its outer end 66 in recess 44 and its inner end 64 receivable in one of slots 54 on stem 52 of pulley 26. The stem 52 is coaxially received about stem 32 and cover 60 is interposed between mainspring 28 andpulley 26. The assembly pin 36 is received within the opening 34 of stem 32 and is frictionally secured therein. With the foregoing assembly, it will be seen from a review of FIG. 3 that out ward pull on pull cable 16 as it passes from pulley 26 through passage 42 tends to wind mainspring 28 about stem 52 while release of the pull cable enables it to be drawn toward barrel 24 by the natural bias of mainspring 28.

To maintain the pull wire 16 within slot 42, a pull wire tube 70 is disposed in slot 42. Pull wire tube 70 is retained in slot 42 by crimping the material of ear 38 about the upper or exposed margins of pull wire tube 70. When fabricating the present appliance, pull wire 16 is passed through the pull wire tube 70.

As illustrated in FIG. 4, a pull wire stop 72 is carried by pull wire 16 outwardly of barrel 24 and spaced back from the end thereof coupled to the tooth undergoing correction to limit the inward extent of pull wire 16 in barrel housing 24.

From the foregoing description, it will be appreciated that the degree of prewind of the mainspring can be predeterminedfor example from one to approximately a half revolution of travel of the pulley 26. Accordingly, when the mainspring has been prewound a predetermined number of revolutions thereby applying a predetermined force to the pull wire, the stop 72 is applied about pull wire 16 such that mainspring 28 cannot totally unwind. That is, a predetermined force is applied to pull wire 16 since the magnitude of the prewind and the sizeof the mainspring force are known. Consequently, pull wire 16 can be drawn from barrel 24 a small discrete distance, preferably a linear distance approximately equal to half a revolution of pulley 26, and over which distance a substantially near constant force is applied to pull wire 16.

FIG. 1 illustrates the pull wire 16 being connected directly to the band 22 clamped about the tooth l8 undergoing correction. It will be appreciated that the end of pull wire 16 outwardly of pull wire stop 72 can be provided in the form of a loop for connection with an end of another wire, the opposite end of which is adapted for connection to the band about the tooth undergoing correction.

To utilize the orthodontic appliance hereof, the orthodontic bands 20 and 22 are applied about the anchor tooth 14 and tooth 18 undergoing correction, respectively. The anchor wire is coupled to the band 20 and the appliance l0 and the pull wire may either be directly coupled to the band 22 about the tooth undergoing correction or connected to another wire, not shown, the opposite end of which is adapted for connection to the band about the tooth undergoing correction. When connecting the appliance 10, it will be appreciated that the pull wire is withdrawn from appliance 10 for approximately one-half a revolution. As

will be recalled, the pull wire is pretensioned before appliance is applied intraorally with the stop 72 butting the housing of the appliance and preventing total unwinding of the mainspring 28. Extension of the pull wire for approximately one-half revolution upon application of the appliance causes a slight increase in the tension applied to the band about the tooth undergoing correction. That is, when the appliance is applied intraorally, the anchor and pull wires 12 and 16 are tensioned between the anchor tooth l4 and tooth l8 undergoing correction and to a slightly greater extent than when the pull wire stop engages the appliance housing. Consequently, the main spring applies a substantially near constant tension force to the tooth undergoing correction during the limited range of linear travel of the pull wire, i.e., a range of linear travel corresponding to approximately one-half revolution of the pulley. It will be appreciated that sets of the appliances 10 are provided with different degrees of prewind on the main spring and/or different main spring sizes whereupon the tension provided the tooth undergoing correction can be selected from a range of various appliances.

In an effort to substantially minimize the force gradi ent applied by the appliance over the limited range of travel of the pull wire and thereby approach a constant tension force on the pull wire and on the tooth undergoing correction throughout the range of movement of the pull wire, the appliance may be modified as illustrated in FIGS. 5-7. To accomplish this, the pulley groove constitutes a cam surface which increases in radius as the pulley rotates to draw the pull wire into the appliance. That is, as the torque of the main spring decreases in winding the pull wire, the effective radius or lever arm also decreases proportionately such that the force or tension on the pull wire remains substantially constant throughout the range of movement of the pull wire for approximately one-half a revolution. Conversely, when the pull wire is extended from the appliance, the torque of the main spring increases proportionately as the lever arm increases to achieve a substantially constant force throughout the range of extending movement of the pull wire.

To accomplish the foregoing, and in a preferred embodiment hereof, the pulley groove is formed slightly eccentric with respect to the center or axis of rotation of the pulley. For example, and as illustrated in FIG. 5, the pulley 26A has a center of rotation designated 100 whereas the circular pulley groove 48a has a center designated 102. Thus, prior to withdrawing the pull wire, the radius R1 from the center of the pulley to a point along the pulley groove 48a from which the pull wire 160 extends tangentially is less than the radius R2 which extends from the center of the pulley to the diametrically opposed location along the pulley groove 48a. Consequently, as the pull wire 16a is pulled from the housing 240, it will be appreciated that the torque on the main spring increases while simultaneously the radius or lever arm increases from a radius R1 to a radius R2. As a result, the force applied by the main spring to the pull wire 16a remains substantially constant as the pull wire is pulled from the housing for approximately one-half of a revolution. This is illustrated from a review of FIGS. 6 and 7 wherein the pull wire 16a is illustrated as extending from the groove 46a in a tangential direction with a force F and at a radius R1 in FIG. 6 while the pulley is illustrated in FIG. 7 after [80 of rotation with the pull wire 16a being illustrated extending tangentially from the groove 48a along a force line designated F and at a radius R2. The radii R1 and R2 are designated by the lever arm distances as L and L+. Consequently, it will be appreciated that once the pull wire 16a is attached to the band about the tooth undergoing correction, the torque on the main spring, as the pull wire is retracted, decreases together with the radius or lever arm. Since the force applied by the spring to the pull wire is proportional to the torque divided by the lever arm. it will be seen that a decrease in torque simultaneously with a proportionate decrease in the lever arm or radius provides a substantially constant force throughout the range of movement of the pull wire corresponding to approximately 180 of rotation of the pulley.

It will be appreciated that the effective cam surface provided by eccentrically locating the center of the circular pulley groove relative to the axis of rotation ofthe pulley can be accomplished by spirally shaping the pulley groove rather than providing a circular pulley groove. However, spirally shaping the groove is a complicated machining operation whereas the provision of both circular pulleys and pulley grooves with centers offset one from the other is a relatively simple machining operation.

For a pulley groove wherein R1 and R2 vary from 0.08 to 0.0850 inches during 180 of movement. the following Table lists the values for the pulling force (ounces) and torque (inch/pounds) for a typical mainspring as the pulley is rotated from 0 to l represented by values of [3.

T Torque Inch/ounces F Pulling force (ounces) L Lever arm (inches) ,8 Angular position of pulley from initial position wherein the pull wire extends tangentially from a radius R] *Torquc values from a typical mainspring Note that throughout the range of movement for a half revolution of the pulley, the pulling force remains substantially constant and in this representative embodiment varies only by 1.6 percent for the motion of the pulley.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embracted therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. An orthodontic appliance comprising a housing, a pulley rotatably carried by said housing, a spring within said housing and secured at one end to said housing and at its opposite end to said pulley to bias said pulley for rotation in one direction, a pull cable wound about said pulley and adapted for extension from an initial position relative to said housing substantially tangentially thereof and against the bias of said spring and for connection with a tooth, means carried by said appliance for maintaining said spring under tension when said pull cable lies in said initial position whereby a tension force of at least a predetermined magnitude is maintained on said cable upon extension thereof from said housing, a guide carried by said housing for directing said cable from said housing substantially tangentially of said pulley, and means carried by said housing for securing the same to another tooth. said securing means being located along substantially the same tangent as said cable when said cable is extended from said housing and on the opposite side of said housing from the pull cable. the effective diameter of the pulley being of the same order as the width of a tooth.

2. An appliance according to claim 1 wherein said guide comprises a tube. and means for securing said tube to said housing.

3. An appliance according to claim 1 wherein said securing means includes a wire coupled at one end to said housing and adapted at its opposite end for connection with an anchor tooth, a band adapted for securement to a tooth undergoing correction, means for connecting the distal end of the cable to said band whereby said housing and the spring and pulley carried thereby are suspended by said wire and said pull cable intermediate the anchor tooth and the tooth undergoing correction.

4. An appliance according to claim 3 including means for limiting the extension of said cable from said housing to less than one revolution of said pulley whereby said spring maintains said cable under a substantially near constant tension force throughout the range of movement of said cable from its initial position to its extended position.

5. An appliance according to claim 3 wherein said wire extends from said housing along substantially the same tangent as said cable when said cable is extended from said housing and on the opposite side of said housing from said pull cable. i

6. An appliance according to claim l including means for limiting the extension of said cable from said housing to less than one revolution of said pulley whereby said spring maintains said cable under a substantially near constant tension force throughout the range of movement of said cable from its initial position to its extended position.

7. An appliance according to claim 6 including a guide carried by said housing for directing said cable from said housing substantially tangentially of said pulley, means carried by said housing for securing the same to another tooth, said securing means being located along substantially the same tangent as said cable 8 when said cable is extended from said housing and on the opposite side of said housing from the pull cable.

8. An appliance according to claim 6 wherein said maintaining means includes a stop carried by said pull cable for engagement against said housing in the initial position of said pull cable to preclude winding said cable on said pulley.

9. An orthodontic appliance comprising a housing, a pulley rotatably carried by said housing; a spring within said housing and secured at one end to said housing and at its opposite end to said pulley to bias said pulley for rotation in one direction, a pull cable wound about said pulley and adapted for extension from said housing against the bias of said spring into a first extended position relative to said housing for connection with a tooth. said pull cable being movable into a second position relative to said housing upon winding said pull cable about said pulley from its first position in response to correcting movement of the tooth and the bias of said spring, said spring being of a type for applying a tension force to said pull cable in said first position thereof of a magnitude different than the magnitude of tension force applied to said pull cable in said second position thereof whereby a force gradient is applied to said pull cable and the tooth undergoing correction over the range of movement of said pull cable between its first and second positions, and means for minimizing the force gradient applied to said pull cable by said spring over the range of movement of said pull cable between its first and second positions whereby a substantially constant force is applied to the pull cable by the spring during movement of the pull cable between its first and second positions.

10. An appliance according to claim 9 including a cam surface about the periphery of said pulley and engageable with said pull cable, said cam surface being defined by surface portions having increasing radii from the center of said pulley to the location of the pull cable as it leaves the pulley when the pulley is wound in a direction increasing the bias applied thereto by the spring whereby the tension force on said pull cable exerted by said spring remains substantially constant throughout said range of movement of said pull cable.

11. An appliance according to claim 9 wherein said pulley has a surface about which the pull cable is wrapped. said surface being defined at least in part by the arc of a circle having a center offset from the axis of rotation of said pulley such that the radius from the center of rotation of the pulley to the point of tangency of the pull cable on the pulley surface is smaller in the initial position of the pulley than in a pulley position with the pull cable extended whereby the tension force on the pull cable exerted by said spring remains substantially constant throughout said range of movement of the pull cable. 1 

1. An orthodontic appliance comprising a housing, a pulley rotatably carried by said housing, a spring within said housing and secured at one end to said housing and at its opposite end to said pulley to bias said pulley for rotation in one direction, a pull cable wound about said pulley and adapted for extension from an initial position relative to said housing substantially tangentially thereof and against the bias of said spring and for connection with a tooth, means carried by said appliance for maintaining said spring under tension when said pull cable lies in said initial position whereby a tension force of at least a predetermined magnitude is maintained on said cable upon extension thereof from said housing, a guide carried by said housing for directing said cable from said housing substantially tangentially of said pulley, and means carried by said housing for securing the same to another tooth, said securing means being located along substantially the same tangent as said cable when said cable is extended from said housing and on the opposite side of said housing from the pull cable, the effective diameter of the pulley being of the same order as the width of a tooth.
 2. An appliance according to claim 1 wherein said guide comprises a tube, and means for securing said tube to said housing.
 3. An appliance according to claim 1 wherein said securing means includes a wire coupled at one end to said housing and adapted at its opposite end for connection with an anchor tooth, a band adapted for securement to a tooth undergoing correction, means for connecting the distal end of the cable to said band whereby said housing and the spring and pulley carried thereby are suspended by said wire and said pull cable intermediate the anchor tooth and the tooth undergoing correction.
 4. An appliance according to claim 3 including means for limiting the extension of said cable from said housing to less than one revolution of said pulley whereby said spring maintains said cable under a substantially near constant tension force throughout the range of movement of said cable from its initial position to its extended position.
 5. An appliance according to claim 3 wherein said wire extends from said housing along substantially the same tangent as said cable when said cable is extended from said housing and on the opposite side of said housing from said pull cable.
 6. An appliance according to claim 1 including means for limiting the extension of said cable from said housing to less than one revolution of said pulley whereby said spring maintains said cable under a substantially near constant tension force throughout the range of movement of said cable from its initial position to its extended position.
 7. An appliance according to claim 6 including a guide carried by said housing for directing said cable from said housing substantially tangentially of said pulley, means carried by said housing for securing the same to another tooth, said securing means being located along substantially the same tangent as said cable when said cable is extended from said housing and on the opposite side of said housing from the pull cable.
 8. An appliance according to claim 6 wherein said maintaining means includes a stop carried by said pull cable for engagement against said housing in the initial position of said pull cable to preclude winding said cable on said pulley.
 9. An orthodontic appliance comprising a housing, a pulley rotatably carried by said housing, a spring within said housing and secured at one end to said housing and at its opposite end to said pulley to bias said pulley for rotation in one direction, a pull cable wound about said pulley and adapted for extension from said housing against the bias of said spring into a first extended position relative to said housing for connection with a tooth, said pull cable being movable into a second position relative to said housing upon winding said pull cable about said pulley from its first position in response to correcting movement of the tooth and the bias of said spring, said spring being of a type for applying a tension force to said pull cable in said first position thereof of a magnitude different than the magnitude of tension force applied to said pull cable in said second position thereof whereby a force gradient is applied to said pull cable and the tooth undergoing correction over the range of movement of said pull cable between its first and second positions, and means for minimizing the force gradient applied to said pull cable by said spring over the range of movement of said pull cable between its first and second positions whereby a substantially constant force is applied to the pull cable by the spring during movement of the pull cable between its first and second positions.
 10. An appliance according to claim 9 including a cam surface about the periphery of said pulley and engageable with said pull cable, said cam surface being defined by surface portions having increasing radii from the center of said pulley to the location of the pull cable as it leaves the pulley when the pulley is wound in a direction increasing the bias applied thereto by the spring whereby the tension force on said pull cable exerted by said spring remains substantially constant throughout said range of movement of said pull cable.
 11. An appliance according to claim 9 wherein said pulley has a surface about which the pull cable is wrapped, said surface being defined at least in part by the arc of a circle having a center offset from the axis of rotation of said pulley such that the radius from the center of rotation of the pulley to the point of tangency of the pull cable on the pulley surface is smaller in the initial position of the pulley than in a pulley position with the pull cable extended whereby the tension force on the pull cable exerted by said spring remains substantially constant throughout said range of movement of the pull cable. 